Crystal structure of Boc-Leu-Aib-Pro-Val-Aib-Aib-Glu(OBzl)-Gln-Phl × H2O,the C-terminal nonapeptide of the voltage-dependent ionophore alamethicin

Boc-L -Leu-Aib-Pro-Val-Aib-Aib-Glu(OBzl)-Gln-Phl (Boc = t-butyloxycarbonyl, Aib = α-aminoisobutyric acid, Bzl = benzyl, Phl = phenylalaninol), C₅₉H₉₀N₁₀O₁₄, the protected C-terminal nonapeptide with the sequence 12–20 of alamethicin, crystallizes in the orthorhombic space group P2₁2₁2₁ with a = 15.666, b = 16.192, c = 26.876 Å, and Z = 4. The molecular conformation is right-handed helical with three α-(5 → 1 hydrogen bonds) and three β-turns (4 → 1 hydrogen bonds). All but two of the hydrogen bonds are significantly longer than the usual value and show bifurcation to some extent. The α/3-helical nonapeptide molecules are arranged head-to-tail along the a direction. The resulting linear antiparallel chains are linked by a weak intermolecular hydrogen bridge, thus forming a two-dimensional layer structure in the ab plane. The conformation of this nonapeptide is almost identical with that of the corresponding C-terminal part found by x-ray crystallography of the eicosapeptide alamethicin.